In the pulp and paper industries, increasing amounts of caustic are required. The preparation of pure sodium and potassium hydroxides has traditionally been achieved by electrolysis of the corresponding chlorides. The disadvantage of this process is that the hydroxides can only be prepared simulataneously with equimolar amounts of chlorine. Thus, this presupposes that chlorine as well as liquor can be disposed of at the same time, which is not the case on certain markets. The demand for one of the chemicals also strongly varies with the state of the market and great price fluctuations may occur.
The pulp mills for the preparation of bleached chemical pulp have traditionally been large consumers of chlorine as well as of soda lye in their external bleach plants. Soda lye is further used as sulfur-free make-up chemical. During the fifties and sixties, the consumption of sodium hydroxide and chlorine was about the same in harmony with the production conditions of the chlorine-alkali plants.
In the seventies, oxygen bleaching and chlorine dioxide began to replace chlorine in the bleach plants of the pulp mills, which has meant a decreased use of elementary chlorine, whereas the consumption of sodium hydroxide has increased.
In Sweden a further environmentally conditioned decrease of the use of chlorine for pulp bleaching is to be expected, whereas new methods requiring still more sodium hydroxide (e.g., treatment of the pulp with nitrogen oxides prior to oxygen bleaching) are in the process of development.
The imbalance in chlorine and caustic use in the pulp industry for bleached chemical pulps is thus already a problem in certain markets and may become still greater in the future.
Traditionally sodium sulfate (salt cake, Na.sub.2 SO.sub.4) is used as make-up chemical for the losses of sodium and sulfur resulting in the pulp manufacture. Alternatively, sodium hydroxide or any other sulfur-free sodium source as well as elementary sulfur or another sulfur-containing chemical can be used.
By way of background, two basic processes are used in the pulp industry: the kraft or sulfate process and teh sulfite process.
The kraft process is clearly described in G. A. Smook, Handbook for Pulp & Paper Technologists (Altlanta, GA: TAPPI, 1982), pages 66-67 as follows:
White liquor containing the active cooking chemicals, sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S), is used for cooking the chips. The residual black liquor containing the reaction products of lignin solubilization is concentration and burned in the recovery furnace to yield an inorganic smelt of sodium carbonate (Na.sub.2 CO.sub.3) and sodium sulfide. The smelt is dissolved to form green liquor, which is reacted with quick lime (CaO) to convert Na.sub.2 CO.sub.3 into NaOH and regenerate the original white liquor. . . PA0 Most mills maintain a white liquor cooking sulfidity within the range of 25 to 35% (based on TTA). The critical low level for sulfidity is not well defined; but most investigators agree that below 15%, a deterioration in cooking reaction rate and pulp quality would certainly occur. A higher level is maintained to provide a safety margin and allow greater use of makeup chemicals containing sulfur (usually Na.sub.2 SO.sub.4).
The sequential steps in the pulping and recovery process: the cooking and washing steps where the chips introduced are converted to pulp; the separation of black liquor containing alkali liquor, hydrolysis salts and sulfonation products; evaporation and burning of the black liquor and the dilution thereof to form a "green liquor" containing Na.sub.2 CO.sub.3 and Na.sub.2 S; a causticizing step wherein the green liquor is converted with lime to "white liquor," a mixture of NaOH and Na.sub.2 S; and finally a recycling of the white liquor to the cooking and washing steps. A more detailed listing of the steps in the process is given in FIG. 7-3 on p. 69 of the Handbook.
In the sulfite process, as presently practiced, the paper chips are cooked, i.e., digested, in a mixture of sulfurous acid (H.sub.2 SO.sub.3) and a soluble base such as sodium sulfite (Na.sub.2 SO.sub.3). In this process, at the end of the cooking stage, the digester contains solid lignocellulosic material, cooking liquor containing the original inorganic chemicals, dissolved or colloidal lignin fragments, carbohydrate materials and resinous substances. The spent cooking liquor is drained from the pulp and chips, concentrated by evaporation. This concentrate is burned in a reductive furnace to form a product which is smelt consisting predominantly of sodium sulfite (Na.sub.2 S) and sodium carbonate (Na.sub.2 CO.sub.3).
This smelt may be solidified by cooling to form flakes or powders, or quenched and dissolved in water to make a solution with a green color (the so-called "green liquor"). The latter processing step is used almost exclusively and provides intermediate for 80% of the pulping liquor manufacture in the world. Most of it goes into making kraft, that is, sulfate liquor which uses the sulfite of the smelt unchanged. The carbonate, however, is converted into caustic soda by a causticizing step with burnt lime. Before causticization, it is not necessary to separate the sulfide from the carbonate.
On the other hand, where a sodium-based sulfide liquor is required from the green liquor, chemical conversion of the sulfide into sulfite is necessary. The carbonate can be used directly to produce the liquor. For the invention of the sulfite into sulfite, the pH of the green liquor is reduced by releasing H.sub.2 S gas, which is subsequently reacted to form SO.sub.2. The latter is then combined with the carbonate in making the sulfite-pulping liquor.
Today the need for sulfide-free alkali liquor is met chiefly by oxidation of white liquor with oxygen from the air and/or by purchase of extranesous sodium hydroxide. Other alternative processes knwon today for the manufacture of sulfide-free alkali liquor are the so-called Stora process, the Tampella process, the Ebara process, and cooling crystallization. The first three of these processes are described, beginning on pages 284, 288 and 294, respectively, of Ingruber et al., Eds., Pulp and Paper Manufacture, Third Edition, Vol. 4, Sulfite Science & Technolgoy (Atlanta, GA: TAPPI, 1985). All of these processes require the consumption of a considerable amount of sodium hydroxide and most require a causticizing step in order to provide the desired hydroxide solution; i.e., the primary product in the processes is a more or less pure sodium carbonate in solid or dissolved form. The causticizing is either effected in a conventional way by the addition of caustic lime (CaO) or by the addition of an amphoteric metal oxide, e.g., Fe.sub.2 O.sub.3, and then burning and leaching (autocausticizing). In this context, reference is made to U.S. Pat. No. 4,000,264. Both processes comprise several process steps.
The Stora process (cg. K. N. Cederqvist et al., TAPPI 43:8 (1960), pp. 702-706) and the Ebara process (cf. A. Teder, Nordisk Cellulosa 1:2 (1984), p. 12) are today considered as technically and economically unsuitable for the preparation of sulfide-free alkali liquor from liquors of sulfate pulp processes.
The Tampella process comprises the evaporation of the hydrogen sulfide by neutralization of a sulfide-containing alkali solution with flue gas (CO.sub.2). The disadvantage of this method is that the desired end product, i.e., the hydroxide, is first eliminated and then regenerated at a later stage.
In cooling crystallization, the green liquor is cooled down to about 9.degree. C., causing a major part of the sodium carbonate in the green liquor to precipitate as Na.sub.2 CO.sub.3. 10H.sub.2 O. In addition to the need for causticizing in this process a further disadvantage is the energy losses resulting from the cooling.
The need for bleached chemical pulp, that is, pulp having a maximum whiteness, further increases the demand for sodium hydroxide. Though the word "bleaching" is used imprecisely in the paper industry, it is now recognized to include both delignification and brightening. Delignification takes place not only in the digester but in pulp treating processes following digestion, as well as in what is conventionally referred to as the bleach plant. There are a number of options now available for extending delignification below that normally achieved in digestion. Such "extended delignification" is discussed in the article by D. W. Reeve entitled "The Future of Bleaching" in the June 1985 edition of the Tappi Journal. Of the extended delignification discussed by Reeve, modified cooking, oxygen delignification, nitrogen dioxide delignification, and hydrogen peroxide delignification all require the use of sodium hydroxide. In modified kraft pulping, alkali is added at several points during the cook to keep the initial concentration low and raise the concentration near the end of the cook. This technique improves the selectivity and minimizes the damage done to the cellulose; such delignification is frequently referred to as nitrogen oxide and peroxide bleaching. The effect of pretreatment with nitrogen dioxide/oxygen (NOX) is also discussed by Abrahamsson et al. in Svensk Papperstidning, No. 3-1982 85 (1982), pages R27-R32. Table 1 of this article also shows the need for sodium hydroxide in the pretreatment steps.
Bleaching post-cooking is generally carried out in a stepwise sequence utilizing different chemicals and conditions in each stage, with washing carried out between stages. This subject is generally discussed in the Handbook, pages 153-172. As noted on page 154 of this article, the following designations are commonly used to describe these chemical treatments:
______________________________________ Chlorination (C) Reaction with elemental chlorine in acidic medium. Alkaline Extraction (E) Dissolution of reaction products with NaOH. Hypochlorite (H) Reaction with hypochlorite in alkaline solution. Chlorine Dioxide (D) Reaction with ClO.sub.2 in acidic medium. Peroxide (P) Reaction with peroxides in alkaline medium. Oxygen (O) Reaction with elemental oxygen at high pressure in alkaline medium. (D.sub.c) or (C.sub.D) Admixture of chlorine and chlorine dioxide. ______________________________________
This article notes that conventionally chlorine and sodium hydroxide were prepared by the electrolysis of brine; however, as chlorine bleaching became less and less desirable, this process became an inefficient means of generating sodium hydroxide because of the imbalance caused by the excess of chlorine produced. To aggravate the situation, the frequent replacement for chlorine is chlorine dioxide; this latter chemical is conventionally made from sodium chlorate by reduction with such reducing agents as chloride ion or sulfur dioxide in 9N sulfuric acid. Again, to form the sodium chlorate, sodium hydroxide is required. Because of the unstable nature of sodium dioxide, it is desirable that this bleach be made at the site of the pulp and paper plant. The by-products of this reaction include sodium sulfate and sulfuric acid. This residual waste acid is conventionally recycled to the recovery system and supply make up sodium and sulfur.
As recognized above, between each bleaching stage there is an extraction stage. These stages, which remove chlorinated and oxygenated lignin from the system, also require caustic because they must be performed at a high pH, one above 10.8, to achieve complete solubilization.
With regard to oxygen bleaching, again, sodium hydroxide is required. Generally from 3 to 7%, based on weight of pulp, is used to neutralize organic acid reaction products and maintain high alkaline conditions. Other forms of oxygen bleaching, such as those employing peroxide, are in need of caustic soda. The conventional peroxide bleaching includes the addition of hydrogen peroxide, magnesium sulfate, sodium silicate and caustic soda. For the best results, the pH must be adjusted and buffered to about 10.5. Basically, this is done by the addition of the sodium hydroxide and sodium silicate. Accordingly, it will be noted that the evolution of both the kraft and sulfite pulp processes has resulted in the need of more and more sodium hydroxide.
In the pulp mill, the sulfur is present as sulfide after the chemical recovery unit. During recent years, the discharge of sulfur (SO.sub.2 to the air and sulfate to the water) and sodium and potassim to the water have decreased considerably, owing to more severe environmental restrictions. This especially applies to Scandinavia but is expected to be applicable to large parts of the rest of the world within the near future. These environmental demands have resulted in the introduction of so-called oxygen bleaching requiring the use of sulfide-free alkali liquor. In order to be able to use, e.g., white liquor as an internal sulfide-free alkali liquor for oxygen bleaching and also for gas washing, the sulfide content has to be eliminated.